Bipolar Regions Research Articles

Gamma-Free Smoke and Particle Detector Using Tritiated Foils

A smoke and particle ionization detector using tritiated amorphous silicon film as the radiation source is demonstrated. The ion chamber design includes both bipolar and unipolar regions, thereby enhancing the particle detection responsivity. The results show that this gamma-free detector is several-fold to fortyfold more responsive than traditional ionization detectors using Am-241. In addition, this ion chamber can function as a dual detector having both photoelectric and ionization detector responsivities

Model for the Coupled Evolution of Subsurface and Coronal Magnetic Fields in Solar Active Regions

According to Babcock's theory of the solar dynamo, bipolar active regions are Ω-shaped loops emerging from a toroidal field located near the base of the convection zone. In this paper, a mean field model for the evolution of a twisted Ω-loop is developed. The model describes the coupled evolution of the magnetic field in the convection zone and the corona after the loop has fully emerged into the solar atmosphere. Such a coupled evolution is required to fully understand what happens to the coronal and subsurface fields as magnetic flux cancels at polarity inversion lines on the photosphere. The jump conditions for the magnetic field at the photosphere are derived from the magnetic stress balance between the convection zone and corona. The model reproduces the observed spreading of active region magnetic flux over the solar surface. At polarity inversion lines, magnetic flux submerges below the photosphere, but the component of magnetic field along the inversion line cannot submerge, because the field in the upper convection zone is nearly radial. Therefore, magnetic shear builds up in the corona above the inversion line, which eventually leads to a loss of equilibrium of the coronal fields and the lift-off of a coronal flux rope. Fields that submerge are transported back to the base of the convection zone, leading to the repair of the toroidal flux rope. Following Martens and Zwaan, interactions between bipoles are also considered.

How far are we from a ‘Standard Model’ of the solar dynamo?

Over the last few years, dynamo theorists seem to be converging on a basic scenario as to how the solar dynamo operates. The strong toroidal component of the magnetic field is produced in the tachocline, from where it rises due to magnetic buoyancy to produce active regions at the solar surface. The decay of tilted bipolar active regions at the surface gives rise to the poloidal component, which is first advected poleward by the meridional circulation and then taken below the surface to the tachocline where it can be stretched to produce the toroidal component. The mathematical formulation of this basic model, however, involves the specification of some parameters which are still uncertain. We review these remaining uncertainties which have resulted in disagreements amongst various research groups and have made it impossible to still arrive at something that can be called a standard model of the solar dynamo.

Discussion paper: The eigen oscillations of the solar active regions

[1] The standing MHD-waves in the potential magnetic arcades describing the bipolar active regions are investigated. The eikonal method allows us the analytical study of the short waves which are divided into the Alfven and magnetosonic waves. The eigen modes of the magnetic arcades are formed as a result of their reflection from the photosphere. The Alfven mode oscillations of the certain frequency take place on the magnetic surfaces. The fast mode oscillations also take place on some surfaces but they are not magnetic. Each oscillation surface has a discrete set of the eigen frequencies.

Babcock–Leighton models of the solar cycle: Questions and issues

This paper is a review of our current state of understanding of dynamo models of the solar cycle based on the Babcock–Leighton mechanism of poloidal field regeneration by the decay of bipolar active regions. It is organized in the form of “point and counterpoint” discussion of ten issues or topics of contention to be found in the recent literature on these dynamo models. These go from similarities and differences with dynamo models based on mean-field electrodynamics, the role of meridional circulation in setting the predicted form of the sunspot butterfly diagram, constraints brought about by light element abundances, non-linear magnetic backreaction on the driving flows, up to the use of Babcock–Leighton models for predicting solar cycle amplitudes.

Dynamic and Topological Features of Photospheric and Coronal Activities Produced by Flux Emergence in the Sun

We study the emergence of magnetic fields in the solar atmosphere with a focus on photospheric and coronal activities produced by flux emergence. These activities are examined using a three-dimensional MHD simulation of an emerging flux tube. The simulation has been performed for a highly twisted flux tube (highly twisted case) and a weakly twisted one (weakly twisted case). The emerging flux tube globally forms a bipolar region associated with a flow in the photosphere. We decompose the flow into several fundamental flow components, such as rotation, expansion/contraction, and distortion. The analysis shows that as the emerging field becomes vertical, a torsional flow appears in a polarity region with intense flux, and the polarity region apparently rotates opposite to the torsional flow. We also find that the evolution of the photospheric neutral line in the highly twisted case is different from that in the weakly twisted case. We then study the coronal structure of an emerging field by focusing on the distribution of current density at the chromospheric footpoint of emerging field lines. Field lines with high current density at their footpoints display a distinct shape in both highly twisted and weakly twisted cases: the highly twisted flux tube produces the confined, sigmoidal structure of coronal loops, while the expanded loop structure appears in the weakly twisted case. Based on these results we discuss the mechanism for producing several solar phenomena, such as filament channels, flux ropes, and sigmoids.

Magnetic flux transport on active cool stars and starspot lifetimes

Context. Many rapidly rotating cool stars show signatures of large magnetic regions at all latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes of 1 month or less, as indicated by observations using (Zeeman-) Doppler imaging techniques. Aims. We aim to estimate the lifetimes of bipolar magnetic regions and starspots on the surfaces of cool stars. We consider different possible configurations for starspots and compare their flux variations and lifetimes based on a magnetic flux transport model. Methods. We carry out numerical simulations of the surface evolution of bipolar magnetic regions (BMRS) and magnetic spots on stars, which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to convective flow patterns. We calculate the flux evolution and the lifetimes of BMRs and unipolar starspots, varying the emergence latitude, surface shear rate, and tilt angle. Results. For BMRs comparable to the largest observed on the Sun, we find that varying the surface flows and the tilt angle modifies the lifetimes over a range of one month. For very large BMRs (area ∼ 10% of the stellar surface) the assumption of a tilt angle increasing with latitude leads to a significant increase of lifetime, as compared to the case without tilt. Such regions can evolve to polar spots that live more than a year. Adopting the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. Random emergence of six additional tilted bipoles in an activity belt at 60° latitude enhanced the lifetimes of polar caps up to 7 years. We have also compared the evolution and lifetime of monolithic starspots with those of conglomerates of smaller spots of similar total area. We find similar decay patterns and lifetimes for both configurations.

Forecasting coronal mass ejections from line-of-sight magnetograms

We show that the length of strong-gradient, strong-field main neutral line, L SGM, which can be measured from line-of-sight magnetograms such as from SOHO/MDI, is both a measure of active-region nonpotentiality and a useful predictor of an active region's future Coronal mass ejections (CME) productivity. To demonstrate that L SGM is a nonpotentiality measure, we show that it is strongly correlated with a direct measure of nonpotentiality. For an appropriate choice of a threshold value, an active region's measured L SGM can be used as a predictor of whether the active region will produce a CME within a few days after the magnetogram. For our set of 36 Marshall Space Flight CentreMSFC vector magnetograms of bipolar active regions, L SGM is found to have a success rate of 80% for prediction of CME productivity in the 0–2 day window. The development of L SGM as a method of measuring nonpotentiality for forecasting large, fast CMEs from present space-based assets is of value to NASA's space exploration initiative (manned missions to the Moon and Mars).

Postreceptoral adaptation abnormalities in early age-related maculopathy

Age-related maculopathy (ARM) has become the major cause of blindness in the Western World. Currently its pathogenesis and primary site of functional damage is not fully understood but ischemia is believed to play a major role. Early detection and precise monitoring of progression of ARM are main goals of current research due to lack of sufficient treatment options, especially in the dry, atrophic form of this disease. We applied the multifocal electroretinogram (mfERG) that can detect any local functional deficit objectively in the central retina. We recorded two paradigms in early ARM patients, the fast flicker and the slow flash paradigm which both represent fast adaptation processes of the proximal retina but under differing photopic conditions and stimulation rates. By subtracting the waveform responses we extracted a late component in the difference waveform that was significantly reduced in the early ARM group compared to a healthy control group (p < or = 0.05). We propose that this multifocal nonlinear analysis permits the detection of adaptative deficits and provides topographic mapping of retinal dysfunction in early ARM. The difference waveform component we extracted with this novel approach might indicate early functional loss in ARM caused by ischemia in postreceptoral layers such as bipolar cells and inner plexiform regions.

Modeling the Sun's open magnetic flux

Context. The heliospheric magnetic field can be extrapolated from the photospheric field distribution using models based upon various approximations. Such models are required for the reconstruction of the open solar magnetic flux prior to the time of direct measurements on the basis of surface flux transport simulations. Aims. We evaluate the consistency of extrapolation models with direct measurements of the heliospheric magnetic field. Furthermore, we study whether extrapolations on the basis of a surface flux transport model for the photospheric magnetic field reproduce the temporal evolution of the measured near-Earth magnetic field. Methods. We use the potential field source surface (PFSS) model and the current sheet source surface (CSSS) model to extrapolate the heliospheric field on the basis of the Wilcox Solar Observatory (WSO) synoptic maps of the solar surface field from 1976-2005. The results are compared with the near-Earth measurements of the radial heliospheric field and its independence of latitude found with Ulysses. Furthermore, we determine extrapolations on the basis of photospheric flux distributions computed with a surface flux transport code, using as input sunspot group areas from the SOON database. Results. The CSSS model based upon WSO data, with a source surface located at >10 R ⊙ and cusp surface at 1.7 R ⊙ . yields the best agreement with the measurements. The flux transport simulations reproduce the observed surface flux together with the open flux if the tilt angle of emerging bipolar magnetic regions is smaller than commonly assumed, but consistent with sunspot observations. Conclusions. The CSSS model with a source surface in the vicinity of the Alfvenic point of the solar wind leads to a good extrapolation of the heliospheric field from solar surface data. A surface flux transport model based upon sunspot data with consistently calibrated tilt angles reproduces the observed evolution of the solar total open flux.

MHD waves in coronal magnetic arcades

We analyze the eigenmodes of the solar coronal magnetic arcade that describes the magnetic field of a bipolar active region using the eikonal method for ideal magnetohydrodynamic equations. We write out the eikonal equations for Alfven and magnetoacoustic waves and derive the equations for the amplitudes of the zeroth approximation. We construct the wave fields for Alfven and fast magnetoacoustic modes and derive the expressions for the eigenfrequencies. We show that Alfven modes of a given frequency are near a number of magnetic surfaces, while fast magnetoacoustic eigenmodes are near nonmagnetic surfaces. A discrete set of eigenfrequencies that continuously change from one surface to another corresponds to each such surface.

First observation of bald patches in a filament channel and at a barb endpoint

Context. The 3D magnetic field topology of solar filaments/prominences is strongly debated, because it is not directly measureable in the corona. Among various prominence models, several are consistent with many observations, but their related topologies are very different. Aims. We conduct observations to address this paradigm. We measure the photospheric vector magnetic field in several small flux concentrations surrounding a filament observed far from disc center. Our objective is to test for the presence/absence of magnetic dips around/below the filament body/barb, which is a strong constraint on prominence models, and that is still untested by observations. Methods. Our observations are performed with the THEMIS/MTR instrument. The four Stokes parameters are extracted, from which the vector magnetic fields are calculated using a PCA inversion. The resulting vector fields are then deprojected onto the photospheric plane. The 180 ◦ ambiguity is then solved by selecting the only solution that matches filament chirality rules. Considering the weakness of the resulting magnetic fields, a careful analysis of the inversion procedure and its error bars was performed, to avoid over-interpretation of noisy or ambiguous Stokes profiles. Thanks to the simultaneous multi-wavelength THEMIS observations, the vector field maps are coaligned with the Hα image of the filament. Results. By definition, photospheric dips are identifiable where the horizontal component of the magnetic field points from a negative toward a positive polarity. Among six bipolar regions analyzed in the filament channel, four at least display photospheric magnetic dips, i.e. bald patches. For barbs, the topology of the endpoint is that of a bald patch located next to a parasitic polarity, not of an arcade pointing within the polarity. Conclusions. The observed magnetic field topology in the photosphere tends to support models of prominence based on magnetic dips located within weakly twisted flux tubes. Their underlying and lateral extensions form photospheric dips both within the channel and below barbs.

Short-Wave Oscillations of Coronal Magnetic Arcades

We investigate MHD waves in potential and force-free magnetic arcades describing bipolar active regions. The eikonal method allows us to study analytically the short waves, which are divided into Alfven and magnetosonic waves. The eigen-modes of magnetic arcades are formed as a result of their reflection at the photosphere. The Alfven mode oscillations of a certain frequency take place on magnetic surfaces. The fast-mode oscillations also take place on some surfaces but they are not magnetic surfaces. Both the Alfven and fast-mode eigen-frequencies change continuously from one such surface to another. Each oscillation surface has a discrete set of eigen-frequencies.

Coronal Magnetic Topologies in a Spherical Geometry I. Two Bipolar Flux Sources

The evolution of the solar corona is dominated to a large extent by the hugely complicated magnetic field which threads it. Magnetic topology provides a tool to decipher the structure of this field and thus help to understand its behaviour. Usually, the magnetic topology of a potential field is calculated due to flux sources on a locally planar photospheric surface. We use a Green's function method to extend this theory to sources on a global spherical surface. The case of two bipolar flux-balanced source regions is studied in detail, with an emphasis on how the distribution and relative strengths of the source regions affect the resulting topological states. A new state with two spatially distinct separators connecting the same two magnetic null points, called the “dual intersecting“ state, is discovered.

Magnetic Causes of Solar Coronal Mass Ejections: Dominance of the Free Magnetic Energy over the Magnetic Twist Alone

We examine the magnetic causes of coronal mass ejections (CMEs) by examining, along with the correlations of active-region magnetic measures with each other, the correlations of these measures with active-region CME productivity observed in time windows of a few days, either centered on or extending forward from the day of the magnetic measurement. The measures are from 36 vector magnetograms of bipolar active regions observed within -30 of disk center by the Marshal Space Flight Center (MSFC) vector magnetograph. From each magnetogram, we extract six whole-active-region measures twice, once from the original plane-of-the-sky magnetogram and again a h r deprojection of the magnetogram to disk center. Three of the measures are alternative measures of the total nonpotentiality of the active region, two are alternative measures of the overall twist in the active-region's magnetic field, and one is a measure of the magnetic size of the active region (the active region's magnetic flux content). From the deprojected magnetograms, we find evidence that (1) magnetic twist and magnetic size are separate but comparably strong causes of active-region CME Productivity, and (2) the total free magnetic energy in an active region's magnetic field is a stronger determinant of the active region's CME productivity than is the field's overall twist (or helicity) alone. From comparison of results from the non-deprojected magnetograms with corresponding results from the deprojected magnetograms, we find evidence that (for prediction of active-region CME productivity and for further studies of active-region magnetic size as a cause of CMEs), for active regions within approx.30deg of disk center, active-region total nonpotentiality and flux content can be adequately measured from line-of-sight magnetograms, such as from SOH0 MDI.

Models of the Large‐Scale Corona. I. Formation, Evolution, and Liftoff of Magnetic Flux Ropes

The response of the large-scale coronal magnetic field to transport of magnetic flux in the photosphere is investigated. In order to follow the evolution on long timescales, the coronal plasma velocity is assumed to be proportional to the Lorentz force (magnetofriction), causing the coronal field to evolve through a series of nonlinear force-free states. Magnetofrictional simulations are used to study the formation and evolution of coronal flux ropes, highly sheared and/or twisted fields located above polarity inversion lines on the photosphere. As in our earlier studies, the three-dimensional numerical model includes the effects of the solar differential rotation and small-scale convective flows; the latter are described in terms of surface diffusion. The model is extended to include the effects of coronal magnetic diffusion, which limits the degree of twist of coronal flux ropes, and the solar wind, which opens up the field at large height. The interaction of two bipolar magnetic regions is considered. A key element in the formation of flux ropes is the reconnection of magnetic fields associated with photospheric flux cancellation at the polarity inversion lines. Flux ropes are shown to form both above the external inversion line between bipoles (representing type B filaments) and above the internal inversion line of each bipole in a sigmoid shape. It is found that once a flux rope has formed, the coronal field may diverge from equilibrium with the ejection of the flux rope. After the flux rope is ejected, the coronal field once again relaxes down to an equilibrium. This ability to follow the evolution of the coronal fields through eruptions is essential for future full-Sun simulations in which multiple bipoles are evolved for many months or years.

Mid-Term Periodicities in the Solar Magnetic Flux

In this paper we report an analysis of the solar magnetic fluxes estimated in the period 1971–1998. We applied the wavelet technique to find the significant periodicities of these series. We concentrate particularly in the mid-term quasi-periodicities (1–2 years). The mid-term periodicity of 1.7 year is the dominant fluctuation for all the types of fluxes analyzed (total, closed, open, low and high latitude open fluxes) and has a strong tendency to appear during the descending phase of solar activity. The mid-term fluctuation of 1 year is significantly present in total and closed fluxes, but it is less important in open fluxes. It is recognizable in the high latitude open flux, but it is absent in the low latitude open flux. Due to the uncertainties involved in estimating the exact period of the quasi-annual peak, this component may not be different from the previously-reported 1.3 year periodicity. The high frequency fluctuations of all the fluxes but the high latitude open flux are in phase with the 11 years solar cycle. The high latitude flux tends to be present all the time, showing that along the cycle both the low latitude bipolar active regions and the polar coronal holes regulate this flux. These findings rule out the possibility of a more basic periodicity different from the 11 years cycle.

Role of Sunspot and Sunspot-Group Rotation in Driving Sigmoidal Active Region Eruptions

We study active region NOAA 9684 (N06L285) which produced an X1.0/3B flare on November 4, 2001 associated with a fast CME (1810 km s−1) and the largest proton event (31 700 pfu) in cycle 23. SOHO/MDI continuum image data show that a large leading sunspot rotated counter-clockwise around its umbral center for at least 4 days prior to the flare. Moreover, it is found from SOHO/MDI 96 m line-of-sight magnetograms that the systematic tilt angle of the bipolar active region, a proxy for writhe of magnetic fluxtubes, changed from a positive value to a negative one. This signifies a counter-clockwise rotation of the spot-group as a whole. Using vector magnetograms from Huairou Solar Observing Station (HSOS), we find that the twist of the active region magnetic fields is dominantly left handed (αbest = −0.03), and that the vertical current and current helicity are predominantly negative, and mostly distributed within the positive rotating sunspot. The active region exhibits a narrow inverse S-shaped Hα filament and soft X-ray sigmoid distributed along the magnetic neutral line. The portion of the filament which is most closely associated with the rotating sunspot disappeared on November 4, and the corresponding portion of the sigmoid was observed to erupt, producing the flare and initiating the fast CME and proton event. These results imply that the sunspot rotation is a primary driver of helicity production and injection into the corona. We suggest that the observed active region dynamics and subsequent filament and sigmoid eruption are driven by a kink instability which occurred due to a large amount of the helicity injection.

Large-scale source regions of earth-directed coronal mass ejections

Based on SOHO/MDI, EIT, Yohkoh/SXT, Hα , and other relevant observations, we analyzed all the earth-directed halo coronal mass ejections (CMEs) in the interval from Mar. 1997 to Dec. 2003. A total of 288 earth-directed CMEs were studied and their associated surface activity events identified. Unlike the previous studies that often attributed a surface activity event or a given active region to a CME source region, this statistical analysis puts emphasis on the large-scale magnetic structures of CMEs, in which the CME-associated surface activity takes place. All the CMEs are found to be associated with large-scale source structures. The identified large-scale structures can be grouped into four different categories: extended bipolar regions (EBRs), transequatorial magnetic loops, transequatorial filaments and their associated magnetic structures, and long filaments along the boundaries of EBRs. The relative percentages of their associated CMEs are 36%, 40%, 13%, and 11%, respectively. The analysis indicates that CMEs are intrinsically associated with source magnetic structures on a large spatial scale.

The Three‐dimensional Interaction between Emerging Magnetic Flux and a Large‐Scale Coronal Field: Reconnection, Current Sheets, and Jets

Using MHD numerical experiments in three dimensions, we study the emergence of a bipolar magnetic region from the solar interior into a model corona containing a large-scale, horizontal magnetic field. An arch-shaped concentrated current sheet is formed at the interface between the rising magnetized plasma and the ambient coronal field. Three-dimensional reconnection takes place along the current sheet, so that the corona and the photosphere become magnetically connected, a process repeatedly observed in recent satellite missions. We show the structure and evolution of the current sheet and how it changes in time from a simple tangential discontinuity to a rotational discontinuity with no null surface. We find clear indications that individual reconnection events in this three-dimensional environment in the advanced stage are not one-off events, but instead take place in a continuous fashion, with each field line changing connectivity during a finite time interval. We also show that many individual field lines of the rising tube undergo multiple processes of reconnection at different points in the corona, thus creating photospheric pockets for the coronal field. We calculate global measures for the amount of subphotospheric flux that becomes linked to the corona during the experiment and find that most of the original subphotospheric flux becomes connected to coronal field lines. The ejection of plasma from the reconnection site gives rise to high-speed and high-temperature jets. The acceleration mechanism for those jets is akin to that found in previous two-dimensional models, but the geometry of the jets bears a clear three-dimensional imprint, having a curved-sheet appearance with a sharp interface to the overlying coronal magnetic field system. Temperatures and velocities of the jets in the simulations are commensurate with those measured in soft X-rays by the Yohkoh satellite.